Hitherto, titanium dioxide (TiO2) (referred to simply as “titanium oxide” in the present specification and appended claims) has been known to be a substance exhibiting a photocatalytic function. Since the 1970's, attempts have been made to form a titanium oxide film on the surface of metallic titanium. Known titanium oxide film formation methods include a method in which a titanium oxide film is formed on the surface of metallic titanium through anodization; a method in which a titanium oxide film is thermally formed on the surface of a metallic titanium sheet in an electric furnace supplied with oxygen; and a method in which a metallic titanium sheet is heated in the combustion flames of city gas at 1,100 to 1,400° C., to thereby form a titanium oxide film on the surface of the metallic titanium sheet (see Non-Patent Document 1).
In order to produce a photocatalytic product exhibiting deodorant, antibacterial, antifog, or antifouling effect by virtue of its photocatalytic function, generally, a substrate is coated with titanium oxide sol through spray coating, spin coating, dipping, or a similar technique, thereby forming a film (see, for example, Patent Documents 1 to 3). However, the formed coating film is likely to undergo exfoliation or wear, and therefore the thus-produced product is difficult to use over a long period of time. Meanwhile, there has been known a method for forming a photocatalytic coating through sputtering (see, for example, Patent Documents 4 and 5).
As has also been known, when crystal nuclei produced through any process (e.g., CVD or PVD) are placed in a sol containing an inorganic or organic metal compound, to thereby grow titanium oxide crystals from the crystal nuclei, or when a sol is applied to the crystal nuclei, followed by solidification and thermal treatment, to thereby grow titanium oxide crystals from the crystal nuclei, the thus-grown titanium oxide crystals assume a columnar shape, and exhibit high photocatalytic activity (see, for example, Patent Documents 6 to 8). However, in such a case, columnar crystals are grown only from seed crystals provided on a substrate, and thus the thus-grown columnar crystals exhibit insufficient bonding strength to the substrate. Therefore, the thus-produced photocatalyst is not necessarily satisfactory in terms of durability (e.g., wear resistance).
Meanwhile, in order to exhibit a photocatalytic function, titanium oxide must receive an UV ray having a wavelength of 400 nm or less. Thus, many studies have been carried out on doping titanium oxide with various elements so as to realize a titanium oxide photocatalyst which functions by visible light. For example, there has been reported that a nitrogen-doped titanium oxide sample serves as an excellent visible-light-responsive catalyst among titanium oxide samples each doped with F, N, C, S, P, Ni, or a like element (see Non-Patent Document 2).
There have also been proposed photocatalysts formed from titanium oxide doped with other elements; for example, a titanium compound in which oxygen sites are substituted with other atoms such as nitrogen, a titanium compound doped with other elements such as nitrogen in an interstitial space of the compound, and a titanium compound in which atoms such as nitrogen are introduced into the grain boundaries of a mass of titanium oxide polycrystals (see, for example, Patent Documents 9 to 12). However, these photocatalysts exhibit unsatisfactory durability, such as poor wear resistance.    Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. 09-241038    Patent Document 2: Japanese Patent Application Laid-Open (kokai) No. 09-262481    Patent Document 3: Japanese Patent Application Laid-Open (kokai) No. 10-053437    Patent Document 4: Japanese Patent Application Laid-Open (kokai) No. 11-012720    Patent Document 5: Japanese Patent Application Laid-Open (kokai) No. 2001-205105    Patent Document 6: Japanese Patent Application Laid-Open (kokai) No. 2002-253975    Patent Document 7: Japanese Patent Application Laid-Open (kokai) No. 2002-370027    Patent Document 8: Japanese Patent Application Laid-Open (kokai) No. 2002-370034    Patent Document 9: Japanese Patent Application Laid-Open (kokai) No. 2001-205103    Patent Document 10: Japanese Patent Application Laid-Open (kokai) No. 2001-205094    Patent Document 11: Japanese Patent Application Laid-Open (kokai) No. 2002-95976    Patent Document 12: Pamphlet of WO01/10553    Non-Patent Document 1: A. Fujishima, et al., J. Electrochem. Soc. Vol. 122, No. 11, pp. 1487-1489, November 1975    Non-Patent Document 2: R. Asahi et al., Science Vol. 293, Jul. 13 (2001), pp. 269-271